Think your dahlias are finished? Think again. Summer heat often causes these late-season favourites to take a temporary pause, with blooms disappearing and buds dropping just when you expect them to be at their best.
Many gardeners assume the display is over and ease off on care, only to miss the beautiful flush of flowers that can arrive in September.
"The best care for dahlias is consistent deadheading and cutting for flowers," says Dave Smith, director of estate gardens and grounds at The Inn, a renowned 10-hectare estate garden.
The secret isn't replacing plants or feeding them endlessly. It's staying consistent with these four simple jobs.
1. Deadhead regularly
Deadheading is non-negotiable if you want dahlias flowering into autumn. "In late summer, check your dahlias every two to three days and deadhead as needed," says Angelika Zaber, Gardening Expert at Online Turf. "It'll give you the best results and the most blooms."
Once a flower fades, dahlias switch their energy to producing seed rather than new blooms. Removing spent flowers interrupts that process, encouraging the plant to keep flowering instead.
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Remove faded blooms by cutting just above the first set of healthy leaves below the flower head. This encourages new shoots and more blooms from that point. Cutting blooms regularly for the house works just as well – a vase arrangement serves the same purpose as deadheading, signalling to the plant to keep producing flowers.
2. Water consistently
Dahlias have large leaves and flowers, so they lose moisture quickly in warm weather. Inconsistent watering slows flowering significantly; drought stress forces plants to prioritise survival over producing new buds.
Water once a week during dry spells, aiming for 2.5cm of water (including rainfall). In containers or light, free-draining soil, you may need to water more frequently.
Always water dahlias at the base to prevent fungal issues. Avoid wetting the leaves and flowers, which can encourage fungal diseases. Mulch that's thinning should be topped up to help retain moisture. A 5cm layer keeps the soil cooler and reduces water loss on hot days.
3. Feed with a high-potash fertiliser
Apply a high-potash liquid fertiliser, such as tomato feed, every one or two weeks whilst flowering. This is the sweet spot – heavier feeding won't produce more blooms and may actually work against you.
High-potash feeds encourage flowering. Just avoid high-nitrogen feeds, which can leave you with plenty of foliage but fewer blooms. Also, always follow the label dose – more is not better. Start feeding once buds form and continue until the first frost. This keeps energy flowing to flower production, not vegetative growth.
4. Stake and support tall plants
By late summer, dahlias become top-heavy with blooms and foliage. Late summer winds can snap unsupported stems, so checking stakes and ties is essential now.
Walk around each plant and gently test stems. If they move more than slightly, the tie is too loose. Replace any ties that have slipped down or become embedded in the stem. Secure tall stems without fastening them too tightly – growth still needs to happen, and over-tight ties will damage the stem. Use soft ties (cloth strips or purpose-made plant ties) rather than twine or wire.
Stems continue to thicken through summer, so check ties every week or two.
Common dahlia problems (and how to fix them)
If your dahlias haven't been deadheaded regularly, the plant has likely shifted its energy to seed production. Start deadheading now, and you should see new flowers within two to three weeks.
Yellowing leaves or slow growth are more often caused by inconsistent watering than a lack of nutrients. If the soil has been drying out between waterings, increase watering before reaching for fertiliser. If moisture isn't the problem, feed weekly with a high-potash fertiliser to encourage fresh buds and flowers.
If tall stems begin to lean under the weight of flowers, check that stakes and ties are still secure. Supporting plants now helps prevent stems snapping in late summer winds.
Maintain the routine until frost
Don't ease off just because summer is drawing to a close. Deadhead, water and feed whilst the plants are actively flowering. Dave recommends keeping up this routine until the first frost.
Once frost has blackened the leaves, cut back the stems and lift the tubers for storage or protect them in the ground, depending on your local climate and soil conditions. The reward? A spectacular September display, when dahlias are often at their very best.
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